Electrosurgical Generator With Adaptive Power Control

ABSTRACT

An electrosurgical generator has an output power control system that causes the impedance of tissue to rise and fall in a cyclic pattern until the tissue is desiccated. The advantage of the power control system is that thermal spread and charring are reduced. In addition, the power control system offers improved performance for electrosurgical vessel sealing and tissue welding. The output power is applied cyclically by a control system with tissue impedance feedback. The impedance of the tissue follows the cyclic pattern of the output power several times, depending on the state of the tissue, until the tissue becomes fully desiccated. High power is applied to cause the tissue to reach a high impedance, and then the power is reduced to allow the impedance to fall. Thermal energy is allowed to dissipate during the low power cycle. The control system is adaptive to tissue in the sense that output power is modulated in response to the impedance of the tissue.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/434,019, filed on May 8, 2003, which is a reissue of U.S. patentapplication Ser. No. 09/209,323, now U.S. Pat. No. 6,228,080, filed onDec. 11, 1998, which is a continuation of U.S. patent application Ser.No. 08/838,548, filed on Apr. 9, 1997, now U.S. Pat. No. 6,033,399, thecontents of which are incorporated herein by reference.

BACKGROUND

1. Field of the Invention

The present invention relates to an electrosurgical generator with anadaptive power control, and more particularly to an electrosurgicalgenerator that controls the output power in a manner that causesimpedance of tissue to rise and fall cyclically until the tissue iscompletely desiccated.

2. Background of the Disclosure

Electrosurgical generators are used by surgeons to cut and coagulatetissue of a patient. High frequency electrical power is produced by theelectrosurgical generator and applied to the surgical site by anelectrosurgical tool. Monopolar and bipolar configurations are common inelectrosurgical procedures.

Electrosurgical generators are typically comprised of power supplycircuits, front panel interface circuits, and RF output stage circuits.Many electrical designs for electrosurgical generators are known in thefield. In certain electrosurgical generator designs, the RE output stagecan be adjusted to control the RMS output power. The methods ofcontrolling the RF output stage may comprise changing the duty cycle, orchanging the amplitude of the driving signal to the RF output stage. Themethod of controlling the RF output stage is described herein aschanging an input to the RF output stage.

Electrosurgical techniques have been used to seal small diameter bloodvessels and vascular bundles. Another application of electrosurgicalenergy is tissue welding. In this application, two layers of tissue aregrasped and clamped together while electrosurgical power is applied. Thetwo layers are thereby welded together. Tissue welding is similar tovessel sealing, except that a vessel or duct is not necessarily sealedin this process. For example, tissue welding may be used instead ofstaples for surgical anastomosis, Electrosurgical power has adesiccating effect on tissue during tissue welding or vessel sealing. Asused herein, the term “electrosurgical desiccation” is meant toencompass any tissue desiccation procedure, including standardelectrosurgical coagulation, desiccation, vessel sealing, and tissuewelding.

One of the problems associated with electrosurgical desiccation isundesirable tissue damage due to thermal effects. The tissue at theoperative site is heated by the electrosurgical current. Healthy tissueadjacent to the operative site can become thermally damaged if too muchheat is allowed to build up at the operative site. The heat may conductto the adjacent tissue and cause a large region of tissue necrosis. Thisis known as thermal spread. The problem of thermal spread becomesimportant when electrosurgical tools are used in close proximity todelicate anatomical structures. Therefore, an electrosurgical generatorthat reduced the possibility of thermal spread would offer a betteropportunity for a successful surgical outcome.

Another problem that is associated with electrosurgical desiccation is abuildup of eschar on the surgical tool, Eschar is a deposit on anelectrosurgical tool that is created from tissue that is desiccated andthen charred by heat. The surgical tools win often lose effectivenesswhen they are coated with eschar. The buildup of eschar could be reducedwhen less heat is developed at the operative site.

Practitioners have known that a measurement of electrical impedance oftissue is a good indication of the state of desiccation of the tissue.Several commercially available electrosurgical generators canautomatically terminate output power based on a measurement ofimpedance. Several methods for determining the optimal point ofdesiccation are known in the field. One method sets a thresholdimpedance, and terminates power once the measured impedance of thetissue crosses the threshold. Another method terminates power based ondynamic variations in the impedance.

A discussion of the dynamic variations of impedance of tissue can befound in the article, Vallfors and Bergdahl “Automatically ControlledBipolar Electrocoagulation,” Neurosurgical Review, 7:2-3, pp. 187-190,1984. FIG. 2 in the Vallfors article shows impedance as a function oftime during heating of tissue. Valfors reports that the impedance valueof tissue proved to be close to minimal at the moment of coagulation.Based on this observation, Vallfors suggests a micro-computer techniquefor monitoring the minimum impedance and subsequently terminating outputpower to avoid charring the tissue.

A second article by Bergdahl and Vallfors, “Studies on Coagulation andthe Development of an Automatic Computerized Bipolar Coagulator,”Journal of Neurosurgery, 75:1, 148-151, July 1991, discusses theimpedance behavior of tissue and its application to electrosurgicalvessel sealing. The Bergdahl article reported that the impedance had aminimum value at the moment of coagulation. The Bergdahl article alsoreported that it was not possible to coagulate safely arteries with adiameter larger than 2 to 2.5 millimeters. The present invention helpsto overcome this limitation by enabling electrosurgical vessel seatingof larger diameter vessels.

U.S. Pat. No. 5,540,684 discloses a method and apparatus forelectrosurgically treating tissue in a manner similar to the disclosuresof Vallfors and Bergdahl. The '684 patent addresses the problemassociated with turning off the RF output automatically after the tissueimpedance has reached a minimum value. A storage device records maximumand minimum impedance values, and an algorithm computes an optimal timefor terminating output power.

U.S. Pat. No. 4,191,188 discloses a variable crest factorelectrosurgical generator. The crest factor is disclosed to beassociated with the coagulation effectiveness of the electrosurgicalwaveform.

U.S. Pat. No. 5,472,443 discloses the variation of tissue impedance withtemperature. The impedance of tissue is shown to fall, and thensubsequently rise as the temperature is increased. The '443 patent showsa relatively lower temperature region (Region A in FIG. 2) where salts,contained within the body fluids, are believed to dissociate, therebydecreasing the electrical impedance. The relatively next highertemperature region (Region B) is where the water in the tissues boilsaway, causing the impedance to rise. The relatively highest region(Region C) is where the tissue becomes charred, resulting in a slightlowering of impedance.

It would be desirable to have an electrosurgical generator that produceda clinically effective output and, in addition, reduced the amount ofheat and thermal spread at the operative site. It would also bedesirable to have an electrosurgical generator that produced a betterquality seal for vessel sealing and tissue welding operations. It wouldalso be desirable to have an electrosurgical generator that desiccatedtissue by applying a minimal amount of electrosurgical energy.

SUMMARY

According to one aspect of the present disclosure an electrosurgicalgenerator for applying output power to tissue is disclosed. Theelectrosurgical generator includes a tissue impedance measurementcircuit configured to measure tissue impedance and a controller coupledto the tissue impedance measurement circuit. The controller is adaptedto cycle output power from the electrosurgical generator to cause acycling of the tissue impedance by applying the output power to tissueand then adjusting the output power to at least one of a lower outputvalue and termination of output power. The controller is further adaptedto re-apply the output power to tissue if tissue impedance does notindicate tissue desiccation and to terminate output power when themeasured tissue impedance indicates tissue desiccation.

According to another aspect of the present disclosure, anelectrosurgical generator for treating tissue by applying energy isdisclosed. The electrosurgical generator includes a desiccation detectorconfigured to determine completeness of tissue desiccation and acontroller coupled to the desiccation detector. The controller isadapted to cycle output power to cause a cycling of tissue impedance inresponse to the degree of tissue desiccation. The controller is furtheradapted to re-apply the output power to tissue if the desiccationdetector does not indicate tissue desiccation.

A method for applying electrosurgical energy to tissue to treat tissueis also contemplated by the present disclosure. The method includes thesteps of: a) cycling output power from an electrosurgical generator tocause a cycling of tissue impedance by applying output power to tissueand then adjusting output power to at least one of a lower output valueand termination of output power and re-applying output power to tissueif tissue impedance does not indicate tissue desiccation. The methodalso includes the steps of c) allowing the tissue impedance to fall to apredetermined minimum value and then raising the output power to causean increase in tissue impedance and d) repeating steps b and c untiltissue impedance at least reaches a predetermined value that correspondsto tissue desiccation.

BRIEF DESCRIPTION OF THE DRAWINGS

Particular embodiments of the present disclosure will be describedherein below with reference to the accompanying drawings. In thefollowing description, well-known functions or constructions are notdescribed in detail to avoid obscuring the present disclosure inunnecessary detail.

FIG. 1 is a block diagram representation of an adaptive oscillatorypower curve according to the present invention.

FIG. 2( a) is a sample of experimental data for a standard vesselsealing operation, showing output power as function of time.

FIG. 2( b) is a sample of experimental data for a standard vesselsealing operation, showing load impedance as a function of time.

FIG. 2( c) is a sample of experimental data for a standard vesselsealing operation, showing output current as a function of time.

FIG. 2( d) is a sample of experimental data for a standard vesselsealing operation, showing output voltage as a function of time.

FIG. 3( a) is a sample of experimental data for an adaptive powercontrol generator, showing output power as fiction of time.

FIG. 3( b) is a sample of experimental data for an adaptive powercontrol generator, showing load impedance as a function of time.

FIG. 3( c) is a sample of experimental data for an adaptive powercontrol generator, showing output current as a function of time.

FIG. 3( d) is a sample of experimental data for an adaptive powercontrol generator, showing output voltage as a function of time.

FIG. 4( a) is a representation of a power curve for a standardelectrosurgical generator.

FIG. 4( b) is a representation of an adaptive oscillatory power curve.

DETAILED DESCRIPTION

The present invention discloses an adaptive, oscillatory power curvewhich is able to reduce thermal spread in each of these areas byapplying power in a cyclical fashion, rather than continuously. Duringthe periods of reduced power application, thermal energy is allowed todissipate which reduces direct thermal conduction. Also, the steam exitsthe weld site in smaller bursts, which produces less thermal damage thanone large burst. Finally, the impedance between the jaws of theelectrosurgical instrument is kept low, which allows current to flowmore directly between the jaws.

Charring is also reduced. High voltages contribute to tissue charring,which is why it is preferable to limit the output voltage of theelectrosurgical generator to 120 volts, and to periodically reduce it toa lower value during power cycling. A relatively low voltage is alsoimportant because it prevents electrical sparks, or arcs, from passingthrough the tissue and burning small holes in the newly sealed, orwelded, tissue.

The transparency, or clarity, at the weld site has been identified as anindicator of successful seal completion. It also gives the surgeonvisual feedback as to whether the seal is a success. Preliminaryfindings indicate that this method may also increase weld sitetransparency. The reason for this is unknown, but it seems reasonablethat reduced charring will allow the weld site to remain moretransparent.

Referring to FIG. 1, a block diagram of an adaptive oscillatory powercontrol system 10 is shown. The line designated by the letter Arepresents the command input signal to the control system 10. Thecommand input signal A is preferably a periodic function, and in stainembodiments the period may vary depending on the dynamics of the tissue.The signal A is representative of the desired tissue impedance. Ameasurement of tissue impedance is represented by line B. A summingblock 11 compares the command input signal A with the measured tissueimpedance B to produce a difference signal C. The summing block 11 maybe comprised of an electrical comparator circuit as is commonly known tocontrol systems engineers.

The difference signal C may be input to a controller 12 that generates acontrol signal D. The control signal D adjusts or terminates the outputpower of the electrosurgical generator by changing the state of the RF.Output Stage 13. The controller 12 may be comprised of an algorithm in amicroprocessor that determines the conditions for power terminationbased on the amplitude of the control signal. Alternatively andequivalently, the controller 12 may be connected directly to themeasured tissue impedance B to terminate power based on the amplitude ofthe measured tissue impedance B. The controller 12 may be comprised ofany combination of proportional integral, and derivative control lawsthat are known to control system engineers. Other types of control laws,such as “bang-bang” control laws, are effective equivalents.

In one embodiment, the command input signal A has a cyclic pattern, forexample a sine wave or a square wave. The cyclic nature of the commandinput signal A causes the control system 10 to regulate the output powerin a cyclic manner to achieve beneficial surgical effects. Thecontroller 12 monitors the difference signal C to determine the responseof the output power E. In one embodiment, when the difference signal Cis large, and the impedance measurement B is above threshold, then thecontroller 12 terminates the output power E.

The control signal D is preferably connected to an R.F. Output Stage 13.The control signal D preferably changes a driving voltage in the R.F.output stage to thereby change the RMS output power from theelectrosurgical generator, shown as line E in FIG. 1. Alternatively andequivalently, the control signal D may change the duty cycle of the R.F.Output Stage 13 thereby effectively changing the RMS output power. Othermeans of changing RMS output power from an R.F. Output Stage, such aschanging current, are known to electrical engineers.

The generator R.F. Output Stage 13 causes the electrosurgical generatorto output a power level E to the tissue 14 of the patient. The tissue 14becomes desiccated, thereby changing the electrical impedance, shown byF in FIG. 1. The electrical impedance F of the tissue is measured by animpedance measurement circuit 15 and reported as the measured tissueimpedance B. The impedance measurement circuit 15 may be any form ofelectrical circuit that measures, or estimates, electrical impedance.The measured tissue impedance B is preferably an electrical signal thatis proportional to the actual tissue impedance F.

Electrical engineers will recognize that output power from anelectrosurgical generator can be adjusted in several ways. For example,the amplitude of the output power can be adjusted. In another example,the output power can be adjusted by changing the duty cycle or the crestfactor. The change or adjustment in output power, as used herein ismeant to refer any change or adjustment in the root mean square (RMS)value of the output power of the electrosurgical generator.

In operation, the control system 10 is designed to cycle the tissueimpedance F for preferably several cycles in order to achieve beneficialeffects. Thus, the command input signal A is a cyclically varying signalsuch as a sine wave. An example of cyclical impedance behavior of tissueis shown in FIG. 3( b). The generator output power that caused thecyclical impedance behavior is shown in FIG. 3( a). The cyclicalbehavior of the present invention can be contrasted with a standardelectrosurgical generator wherein the output power is shown in FIG. 2(a) and the tissue impedance is shown in 2(b).

The present invention discloses an adaptive, oscillatory power curvewhich is able to reduce thermal spread in each of these areas byapplying power in a cyclical fashion, rather than continuously. Duringthe periods of reduced power application, thermal energy is allowed todissipate which reduces direct thermal conduction. Also, the steam exitsthe weld site in smaller bursts, which produces less thermal damage thanone large burst. Finally, the impedance between the jaws of theelectrosurgical instrument is kept low, which allows current to flowmore directly between the jaws.

Charring is thought to be reduced by the present invention. Highvoltages contribute to tissue charring, which is why it is preferable tolimit the output voltage of the electrosurgical generator to 120 volts,and to periodically reduce it to a lower value during power cycling. Arelatively low voltage is also important because it prevents electricalsparks, or arcs, from passing through the tissue and burning small holesin the newly sealed, or welded, tissue.

The transparency, or clarity, at the weld site has been identified as anindicator of successful seal completion. It also gives the surgeonvisual feedback as to whether the seal is a success. Preliminaryfindings indicate that this method may also increase weld sitetransparency. The reason for this is unknown, but it seems reasonablethat reduced charring will allow the weld site to remain moretransparent.

A plot of output power vs. load impedance is called a “power curve.” Arepresentation of a standard power curve is shown in FIG. 4( a). At lowimpedance, the output is typically current limited, and this is shown asthe “constant current” line segment on FIG. 4( a). At midranges ofimpedance, the electrosurgical generator has a power control system thatmaintains the output power at a constant level by adjusting the outputvoltage, as shown by the “constant power” line segment on FIG. 4( a).Eventually, the load impedance becomes large, and the output powercannot be maintained without applying unacceptably high output voltages.Thus, a voltage limit is reached, and the output power drops off becausethe output current is falling and the output voltage is at a limit. Thedrop in output power is shown as the “constant voltage” line segment inFIG. 4( a).

The present invention is related to an electrosurgical generator havingan adaptive oscillatory power curve as shown in FIG. 4( b). The adaptiveoscillatory power curve is produced by a power control system in theelectrosurgical generator. The design details of the control system canbe implemented in several ways which are well known to control systemengineers.

The first part of the adaptive oscillatory power curve, shown at theline segment I in FIG. 4( b), is similar to the standard power curve,wherein the generator applies high current into a low impedance loaduntil a maximum power limit, shown as A, is reached. In the next “leg”of the power curve, shown by line segment B, output current begins tofall, and output voltage begins to rise as the generator adjusts theoutput voltage to maintain constant output power at the level marked byA. The generator then begins looking for signs to indicate the onset ofboiling in the tissue. Such signs include a very rapid rise inimpedance, or a high value of voltage, such as 120 volts. The localmaximum of the impedance curve is shown by letter K in FIG. 4( b). Thedotted line, marked C and labeled V=120 V, shows the possible outputpower if the generator were to maintain a voltage limit of 120 volts,which is a preferred voltage limit. Rather than follow the V=120 V line,a controller in the generator drops the output power. This can beaccomplished, in one embodiment, by dropping the output voltage limit tobetween zero and 70 volts, and preferably 50 volts, as shown in linesegment D. In another embodiment of the control system, the output powercan be reduced by other combinations of output current reduction and/oroutput voltage reduction.

As a consequence of the lower voltage limit, the output power drops tothe level indicated by H in FIG. 4( b). In certain embodiments, H may bezero watts. At this lower output power, desiccation stops and the tissueimpedance starts to fall. A preferred lower voltage limit of 50 voltsmay be used as shown by dotted line E and marked “V=5 volts”. Once theimpedance has reached a local minimum, shown by J, or after a set periodof time, the power control system raises the output power back to levelA, which corresponds to an output voltage limit of 120 volts in thepreferred embodiment. Thus, the output power rises back to level A, andthe impedance rises again, until the onset of boiling or an impedancethreshold is reached. The cyclical portion of the power curveincorporating line segments B, D, and E is an important part of thisinvention and will continue until the tissue is desiccated. When thetissue is desiccated, the power will terminate as shown when impedancereaches point L. In certain embodiments, point L will be substantiallythe same as point K.

The behavior shown in FIG. 4( b) can be observed in FIGS. 3( a), 3(b),3(c) and 3(d). Power oscillations between 120 watts and 20 watts in FIG.3( a) correspond to cyclical movement between power level A and powerlevel H in FIG. 4( b). Impedance oscillations in FIG. 3( b) correspondto cyclical movement between impedance level K and impedance level J inFIG. 4( b). It will be understood by control systems engineers that FIG.4( b) is highly idealized, and the cyclical behavior may not alwaysreach exactly the same local maxima and minima. This can be observed inFIG. 3( a), where the local maxima of the power curve may not alwaysreach 120 volts.

It is theorized by the inventor that the following phenomena occur. Theinitial high output power initiates boiling in the tissues. Thesubsequent low output power is insufficient to maintain boiling, andhence boiling in the tissue stops. After boiling stops, if the tissue isnot completely desiccated then the impedance will fall to a lower value.Next, the low impedance allows output power to increase, which re-heatsthe tissue to the point of boiling. The voltage is also pulled higherduring the process, and remains so until the power curve can sense theonset of boiling, and lower the voltage, preferably back to 50 volts.The process continues until the tissue is fully desiccated. Anoscillation is one cycle of high output power followed by low outputpower.

FIGS. 2( a) through 2(d) show experimental results on tissue samplesusing a standard power curve. FIGS. 3( a) through 3(d) show experimentalresults using an adaptive oscillatory power curve. The general nature ofthe invention can be seen by comparing FIG. 2( a) with FIG. 3( a). FIG.2( a) shows a 100 watt electrosurgical output that is appliedcontinuously to tissue. As the tissue desiccates, the impedance of thetissue rises and the output power in FIG. 2( a) is seen to fall offbelow 20 watts. In contrast, FIG. 3( a) shows an oscillating outputpower that varies from approximately 100 watts to approximately 20watts. The effects on tissue impedance can be seen by comparing FIG. 2(b) with FIG. 3( b). The tissue impedance resulting from the standardpower curve is shown to continuously increase in FIG. 2( b), perhapsafter an initial drop. The tissue impedance resulting from the adaptiveoscillatory power curve is shown to oscillate in FIG. 3( b) and thus hasseveral local minima.

Output voltage and output current show a cyclic behavior in the adaptiveoscillatory power curve. The cyclic behavior is absent in the standardpower curve. FIGS. 2( c) and 3(c) can be compared to show the differencein output current between the standard power curve and the adaptiveoscillatory power curve. In each case the maximum output current risesabove 2 amps RMS. FIGS. 2( d) and 3(d) can be compared to show thedifference in output voltage between the standard power curve and theadaptive oscillatory power curve. A voltage limit, preferably in eachcase 120 volts, prevents arcing that might leave pinholes in the tissueseal.

In one embodiment of the adaptive oscillatory power curve, the generatortemporarily lowers the output voltage limit to 50 volts whenever theoutput voltage reaches 120 volts. This causes a reduction in outputpower, and if the tissue is not completely desiccated, a correspondingsignificant reduction in tissue impedance. After the reduction in tissueimpedance, the output voltage limit is reset to 120 volts, allowing arise in output power. This reduction and subsequent rise in output powerconstitutes a cycle.

Designers of electrosurgical generators have found that impedance is agood indicator of the desiccation state of the tissue. However, skilledartisans will recognize that it may not be necessary to compute an exactvalue for impedance. An electrical measurement that is proportional tothe tissue impedance can be used as a functional equivalent. In oneembodiment, the control system can properly create the adaptiveoscillatory power curve based on measurements of time, and outputvoltage.

Table 1 shows a comparison between two sets of tests which compare astandard power curve with an adaptive oscillatory power curve. Test 1indicates use of the standard power curve, while Test 2 indicates theuse of the adaptive oscillatory power curve. Size indicates the vesseldiameter in millimeters, burst pressures are measured in p.s.i.,sticking, charring, and clarity are subjective measures ranked from 0 to3, (where 0 represents a low value for sticking and charring, and 0represents a poor value for clarity), and ts indicates thermal spread,measured in millimeters.

TABLE 1 Comparison of Standard Power Curve with Adaptive Power CurveTest # samples size bp stick charring clarity ts 1 (mean) 19 2.57 17.260.63 1.11 1.89 2.11 1 (SD) 1.35 1.04 0.76 0.81 1.29 0.74 1 (min) 1 12.961 (max) 6 17.50 2 (mean) 20 2.55 17.39 0.80 0.60 1.95 1.65 2 (SD) 1.360.44 1.06 0.60 1.36 0.81 2 (min) 1 15.52 2 (max) 5 17.50

Table 1 illustrates that the adaptive oscillatory power curve (Test 2)has several advantages over the standard power curve (Test 1). Mostnotable is the lower amount of thermal spread: a mean value of 2.1 mmfor the standard power curve, and 1.65 mm for the adaptive oscillatorypower curve. The subjective measures for sticking, charring and clarityof the weld show that the adaptive oscillatory power curve offerimprovements over the standard power curve.

In general, the invention is an electrosurgical generator for treatingtissue, wherein the electrosurgical generator comprises a circuit forgenerating a measurement of the load impedance, and an output powercontroller having means for inducing multiple oscillations of the loadimpedance in response to the measurement. The load impedance refers tothe impedance of the tissue being treated by the electrosurgicalgenerator. The circuit for generating a measurement of the loadimpedance can be analog or digital, and typically requires an outputvoltage sensor and an output current sensor. The output voltage isdivided by the output current to compute a measurement of loadimpedance.

The means for inducing multiple oscillations of the load impedancepreferably comprises a control system which can selectively control theoutput voltage to cause appropriate oscillations of the output power. Inmany electrosurgical generators, an output power control circuit has anadjustable voltage supply connected to the primary side of an isolationtransformer. The secondary winding of the transformer is connected to anoutput resonant circuit. The voltage supply has an adjuster for changingthe voltage to the transformer, and thereby changing the output voltageof the electrosurgical generator. A digital signal may be used tocontrol the voltage supply.

The means for inducing multiple oscillations preferably comprise afeedback control system, where the feedback is a measurement of the loadimpedance. The control system preferably includes an algorithm in amicroprocessor. The algorithm in the microprocessor can monitor the loadimpedance and determine how the load impedance is responding to a changein the output power.

In the preferred embodiment, the control system sets an output voltagelimit of 120 volts RMS, and then controls the output power to a userdesired setting, for example 100 watts. When the impedance is relativelylow, a high current will combine with an output voltage of less than 120volts to yield the desired power of 100 watts. As the impedance rises,the output current will fall, and the output voltage will be increasedby the circuit to maintain the desired output power. When the voltagelimit of 120 volts is reached, the control system will automaticallylower the output voltage to a low value, preferably 50 volts. Thiseffectively lowers the output power. If the tissue is not completelydesiccated, the lower output power will cause the impedance to dropsignificantly. Once a local impedance minimum is detected, or after aset period of time, the output voltage limit is reset to 120 volts bythe control system, and the cycle repeats. It has been found throughexperimentation that the oscillations of the load impedance will occurin the frequency range of one to twenty hertz, and this range has beenreferred to herein as the thermal bandwidth. In one embodiment, thecontrol system terminates the output power after a set period of timewhich was three seconds. Alternatively, the control system can terminatepower when the impedance reaches a threshold of 2000 ohms. Anotheralternative is to terminate output power when the measurement ofimpedance indicates that the impedance does not substantially fall inresponse to a drop in the output power.

The present invention is applicable to any form of electrosurgicalcoagulation. The benefits of the present invention, including reducedthermal spread, less eschar buildup, and improved desiccation, can beapplied to both monopolar and bipolar electrosurgical generator outputs.While a particular preferred embodiment has been illustrated anddescribed, the scope of protection sought is in the claims that follow.

1. An electrosurgical generator for applying output power to tissue, theelectrosurgical generator comprising: a tissue impedance measurementcircuit configured to measure tissue impedance; and a controller coupledto the tissue impedance measurement circuit, the controller adapted tocycle output power from the electrosurgical generator to cause a cyclingof the tissue impedance by applying the output power to tissue and thenadjusting the output power to at least one of a lower output value andtermination of output power, the controller further adapted to re-applythe output power to tissue if measured tissue impedance does notindicate tissue desiccation and to terminate output power when themeasured tissue impedance indicates tissue desiccation.
 2. The generatoraccording to claim 1, wherein the controller changes the output voltageto cycle the output power.
 3. The generator according to claim 1,wherein the controller changes the output current to cycle the outputpower.
 4. The generator according to claim 1, wherein the output voltageis cycled by lowering the output voltage once the output voltage reachesa predetermined maximum and raising the output voltage if the rise inmeasured tissue impedance does not indicate tissue desiccation.
 5. Thegenerator according to claim 1, wherein the output power is cycled at afrequency that is from about 1 Hz to about 20 Hz.
 6. The generatoraccording to claim 1, wherein the output voltage does not exceed 120volts.
 7. The generator according to claim 1, further comprising acomparator and wherein the measured tissue impedance value is comparedto a first signal representative of a desired tissue impedance value bythe comparator to produce a difference signal.
 8. The generatoraccording to claim 7, wherein the difference signal is input into thecontroller which generates a signal to adjust the power.
 9. Thegenerator according to claim 7, wherein the first signal has a cyclicpattern.
 10. The generator according to claim 9, wherein the firstsignal is a sine wave.
 11. An electrosurgical generator for treatingtissue by applying energy, the electrosurgical generator comprising: adesiccation detector configured to determine completeness of tissuedesiccation; and a controller coupled to the desiccation detector, thecontroller adapted to cycle output power to cause a cycling of tissueimpedance in response to the degree of tissue desiccation, thecontroller further adapted to re-apply the output power to tissue if thedesiccation detector does not indicate tissue desiccation.
 12. Thegenerator according to claim 11, wherein the output power is terminatedby the controller upon detection of desiccated tissue.
 13. The generatoraccording to claim 12, wherein the desiccation detector furthercomprises impedance measuring circuitry adapted to measure tissueimpedance and to determine degree of tissue desiccation based on themeasured tissue impedance.
 14. The generator according to claim 13,wherein the impedance measuring circuitry adjusts the output power byadjusting the output voltage within a predetermined voltage range. 15.The generator according to claim 11, wherein the output power isrepeatedly increased and decreased by the circuitry at a frequency fromabout 1 Hz to about 20 Hz.
 16. A method for applying electrosurgicalenergy to tissue to treat tissue, the method comprising: a) cyclingoutput power from an electrosurgical generator to cause a cycling oftissue impedance by applying output power to tissue and then adjustingoutput power to at least one of a lower output value and termination ofoutput power; b) re-applying output power to tissue if tissue impedancedoes not indicate tissue desiccation; c) allowing the tissue impedanceto fall to a predetermined minimum value and then raising the outputpower to cause an increase in tissue impedance; and d) repeating steps band c until tissue impedance at least reaches a predetermined value thatcorresponds to tissue desiccation.
 17. The method according to claim 16,further comprising the step of: lowering the output voltage once theoutput voltage reaches a predetermined maximum and raising the outputvoltage if the rise in measured tissue impedance does not indicatetissue desiccation.
 18. The generator according to claim 1, wherein thecycling of the output power is accomplished at a frequency from about 1Hz to about 20 Hz.